In ventilating and air conditioning systems for buildings, intimate mixing of airstreams which are introduced at different temperature levels through a common duct is essential in order to avoid undesirable stratification of the air, for example, as a preliminary to its passage into a room air space. This problem is prevalent in intermixing cold outside air with warmer return air from a space, since the respective airstreams will tend to stratify or remain in separate layers and, when introduced into one or more areas or spaces, will exhibit a wide disparity in temperature. Various types of movable damper or mixing valve systems have been devised in the past to minimize or eliminate stratification of the air. Representative of such approaches are those dislcosed in U.S. Pat. Nos. 2,650,535 to P. E. Hord; 2,684,024 to F. J. Kurth et al; 2,872,859 to W. W. Kennedy; 3,212,424 to J. R. Davis; 3,732,799 to H. J. Spoormaker; and 3,973,590 to H. G. Logsdon.
It is desirable to devise a mixing system or apparatus which is capable of intermixing airstreams of different temperature passing through a common duct so as to greatly improve mixing effectiveness and eliminate stratification of the air. U.S. Pat. No. 3,180,245 to Theodore Erickson et al discloses a mixing device in which a plurality of fixed blades or vanes radiate outwardly from a common center and are arranged in adjacent rings with the vanes in each ring pitched in an opposite direction to that of the other ring so as to develop a plurality of oppositely rotating, whirling streams. Although Erickson et al has been found to be effective in reducing stratification and increasing mixing effectiveness of airstreams which would otherwise tend to stratify, it has been found that vastly improved mixing effectiveness can be achieved through the utilization of one or more concentric rows of fixed blades or vanes within a single ring or frame by virtue of a novel and improved blade configuration and arrangement which will establish minimum pressure drop, maximize effectiveness in mixing the airstreams and maintain a uniform or flat velocity profile of the resultant mixed airstreams downstream of the mixing device.